SSD vs HDD: Differences in Speed, Durability and Cost

A hard disk drive (HDD) stores data magnetically on spinning platters read by a moving head. A solid-state drive (SSD) stores data in NAND flash chips with no moving parts. SSDs are many times faster, silent and more resistant to shock; HDDs cost much less per terabyte and reach larger capacities, so the two are often used together: an SSD for the system and active files, and an HDD for bulk storage.

AspectHDDSSD
How it stores dataMagnetic platters spinning under a moving read/write headNAND flash memory chips, with no moving parts
Sequential speedAbout 100 to 250 MB/sAbout 500 MB/s (SATA) up to 14 GB/s (fast NVMe)
Random access and latencySlow: milliseconds for the head to move and the disk to rotateFast: tens of microseconds, and hundreds of thousands of operations per second or more
Shock resistanceVulnerable to drops and vibration while runningResistant, because nothing moves
NoiseAudible spinning and seekingSilent
PowerHigher: a motor and head actuatorLower, especially at idle
Cost per terabyteLowHigher, but falling
CapacityTens of terabytes per driveConsumer drives of 1 to 8 TB; larger enterprise drives
WearMechanical wear; no write-cycle limitLimited program/erase cycles, managed by wear leveling and rated in TBW
Special maintenanceFragmentation slows it downNeeds TRIM and spare area; fragmentation hardly matters
Best forBulk storage, backups, network storage, surveillance and archivesOperating system drives, laptops, phones, gaming, databases

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

SSD
For anything where speed and responsiveness matter: the system drive, applications, laptops and mobile devices.
HDD
For large amounts of rarely accessed data at the lowest cost per terabyte: backups, media libraries, archives.
Both
A common setup is a fast SSD for the system and active work, and a large HDD for storage.

FAQ

Is an SSD always faster than an HDD?

For almost all tasks, yes. SSDs have access times of tens of microseconds versus milliseconds, and sequential speeds from about 500 MB/s up to 14 GB/s, against about 100 to 250 MB/s for an HDD.

Do SSDs wear out?

Flash cells survive a limited number of program/erase cycles. Wear leveling and spare area spread the wear, and drives are rated in TBW, which is far more than most users write.

Why are HDDs still used?

They are much cheaper per terabyte and reach larger capacities, so they remain the choice for bulk storage, backups and archives.

What is TRIM?

A command that tells the SSD which blocks the operating system no longer uses, so garbage collection can erase them in advance and keep write performance up.

Check your understanding

Which correctly describes an SSD compared with an HDD (hard disk drive)?

  1. AIt records magnetically on spinning disks
  2. BIt has no mechanically moving parts, offering fast random access and shock resistance
  3. CIt is always cheaper than an HDD for the same capacity
Show answer and explanation

Answer: B. It has no mechanically moving parts, offering fast random access and shock resistance

An SSD records data in NAND flash and has no moving parts such as platters or heads. An HDD needs several ms per access for head movement and rotational delay, whereas an SSD takes tens to about 100 µs, making random access hundreds of times faster. SSDs are also shock-resistant and silent. HDDs remain cheaper per terabyte and are still used for bulk storage.

What is the purpose of wear leveling, used in SSDs and similar devices?

  1. ATo compress data and increase capacity
  2. BTo encrypt and protect data
  3. CTo spread writes across the whole device instead of concentrating them on certain blocks, extending its life
Show answer and explanation

Answer: C. To spread writes across the whole device instead of concentrating them on certain blocks, extending its life

Each block tolerates a limited number of erase cycles, so writing the same logical address repeatedly would wear one spot out early. With wear leveling, the controller maintains a logical-to-physical mapping (the FTL) and steers writes to blocks with fewer erases. It also occasionally relocates rarely changed static data, evening out wear across all blocks and extending product life.

Which correctly describes garbage collection in an SSD?

  1. AMoving valid data out of blocks that contain invalid data, then erasing those blocks to create free space
  2. BRemoving computer viruses
  3. CCompressing data
Show answer and explanation

Answer: A. Moving valid data out of blocks that contain invalid data, then erasing those blocks to create free space

NAND cannot overwrite in place and erases only by block, so each rewrite leaves the old version behind as an invalid page. The controller picks blocks with many invalid pages, moves their valid data to another block, and erases them to create free space. These extra copies cause write amplification, and the fuller the drive, the more often garbage collection runs and the more write performance suffers.

  1. Which command notifies an SSD of space freed by files deleted by the OS, making garbage collection more efficient?
  2. Which correctly describes "write amplification" in an SSD?
  3. Which correctly describes "overprovisioning" in an SSD?

Practice SRAM and flash memory questions

Key terms explained

NAND flash memory
Non-volatile memory in which memory cells are connected in series. It is dense and easy to scale to large capacities, and is used in SSDs and memory cards.
3D NAND
NAND flash in which memory cells are stacked vertically in many layers to increase capacity. It was introduced to get past the limits of planar scaling, and layer counts now exceed 200.

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